It is generally known that antenna performance is dependent upon the antenna size, shape, and the material composition of certain antenna elements, as well as the relationship between the wavelength of the received or transmitted signal and certain antenna physical parameters (e.g., length for a linear antenna and diameter for a loop antenna). These relationships and physical parameters determine several antenna performance characteristics, including input impedance, gain, directivity, polarization and the radiation pattern. Generally, for an operable antenna, the minimum physical antenna dimension (or the minimum effective electrical length) must be on the order of a quarter wavelength (or a multiple thereof) of the operating frequency, which thereby limits the energy dissipated in resistive losses and maximizes the energy transmitted. Quarter wave length and half wave length antennae are the most commonly used.
The burgeoning growth of wireless communications devices and systems has created a substantial need for physically smaller, less obtrusive, and more efficient antennas that are capable of wide bandwidth or multiple frequency band operation, and/or operation in multiple modes, i.e., selectable signal polarizations or radiation patterns. As the physical enclosures for pagers, cellular telephones and wireless Internet access devices (e.g., PCMCIA cards for laptop computers) shrink, manufacturers continue to demand improved performance, multiple operational modes and smaller sizes for today""s antennae. It is indeed a difficult objective to achieve these features while shrinking the antenna size.
Smaller packaging of state-of-the-art communications devices does not provide sufficient space for the conventional quarter and half wavelength antenna elements. As is known to those skilled in the art, there is a direct relationship between physical antenna size and antenna gain, at least with respect to a single-element antenna, according to the relationship: gain=(xcex2R){circumflex over ( )}2+2xcex2R, where R is the radius of the sphere containing the antenna and xcex2 is the propagation factor. Increased gain thus requires a physically larger antenna, while users continue to demand physically smaller antennas. As a further constraint, to simplify the system design and strive for minimum cost, equipment designers and system operators prefer to utilize antennas capable of efficient multi-frequency and/or wide bandwidth operation. Finally, gain is limited by the known relationship between the antenna frequency and the effective antenna length (expressed in wavelengths). That is, the antenna gain is constant for all quarter wavelength antennas of a specific geometry i.e., at that operating frequency where the effective antenna length is a quarter wavelength of the operating frequency.
One basic antenna commonly used in many applications today is the half-wavelength dipole antenna. The radiation pattern is the familiar donut shape with most of the energy radiated uniformly in the azimuth direction and little radiation in the elevation direction. Frequency bands of interest for certain wireless communications devices include 1710 to 1990 MHz and 2110 to 2200 MHz. A half-wavelength dipole antenna is approximately 3.11 inches long at 1900 MHz, 3.45 inches long at 1710 MHz, and 2.68 inches long at 2200 MHz. The typical gain is about 2.15 dBi.
A derivative of the half-wavelength dipole is the quarter-wavelength monopole antenna placed above a ground plane. The physical antenna length is a quarter-wavelength, but the ground plane creates an effective half-wavelength dipole and therefore the antenna characteristics resemble those of a half-wavelength dipole, that is the radiation pattern shape for the quarter-wavelength monopole above a ground plane is similar to the half-wavelength dipole pattern, with a typical gain of approximately 2 dBi.
The common free space (i.e., not above ground plane) loop antenna (with a diameter of approximately one-third the wavelength) also displays the familiar donut radiation pattern along the radial axis, with a gain of approximately 3.1 dBi. At 1900 MHz, this antenna has a diameter of about 2 inches. The typical loop antenna input impedance is 50 ohms, providing good matching characteristics.
Another conventional antenna is the patch, which provides directional hemispherical coverage with a gain of approximately 3 dBi. Although small compared to a quarter or half wavelength antenna, the patch antenna has a relatively narrow bandwidth.
Given the advantageous performance of a quarter and half wavelength antennas, prior art antennas have typically been constructed with elemental lengths on the order of a quarter wavelength of the radiating frequency with the antenna operated above a ground plane. These dimensions allow the antenna to be easily excited and to be operated at or near a resonant frequency, limiting the energy dissipated in resistive losses and maximizing the transmitted energy. But, as the operational frequency increases/decreases, the operational wavelength decreases/increases and the antenna element dimensions proportionally decrease/increase.
Thus antenna designers have turned to the use of so-called slow wave structures where the structure physical dimensions are not equal to the effective electrical dimensions. Recall that the effective antenna dimensions should be on the order of a half wavelength (or a quarter wavelength above a ground plane) to achieve the beneficial radiating and low loss properties discussed above. Generally, a slow-wave structure is defined as one in which the phase velocity of the traveling wave is less than the free space velocity of light. The wave velocity is the product of the wavelength and the frequency and takes into account the material permittivity and permeability, i.e., c/((sqrt(∈r)sqrt(xcexcr))=xcexf. Since the frequency remains unchanged during propagation through a slow wave structure, if the wave travels slower (i.e., the phase velocity is lower) than the speed of light, the wavelength within the structure is smaller than the free space wavelength. Thus, for example, a half wavelength slow wave structure is shorter than a half wavelength structure where the wave propagates at the speed of light (c). The slow-wave structure de-couples the conventional relationship between physical length, resonant frequency and wavelength. Slow wave structures can be used as antenna elements (e.g., feeds) or as antenna radiating structures.
Since the phase velocity of a wave propagating in a slow-wave structure is less than the free space velocity of light, the effective electrical length of these structures is greater than the effective electrical length of a structure propagating a wave at the speed of light. The resulting resonant frequency for the slow-wave structure is correspondingly increased. Thus if two structures are to operate at the same resonant frequency, as a half-wave dipole, for instance, then the structure propagating the slow wave will be physically smaller than the structure propagating the wave at the speed of light.
Slow wave structures are discussed extensively by A. F. Harvey in his paper entitled Periodic and Guiding Structures at Microwave Frequencies, in the IRE Transactions on Microwave Theory and Techniques, January 1960, pp. 30-61 and in the book entitled Electromagnetic Slow Wave Systems by R. M. Bevensee published by John Wiley and Sons, copyright 1964. Both of these references are incorporated by reference herein.
A transmission line or conductive surface on a dielectric substrate exhibits slow-wave characteristics, such that the effective electrical length of the slow-wave structure is greater than its actual physical length according to the equation,
le=(∈eff1/2)xc3x97lp,
where le is the effective electrical length, lp is the actual physical length, and ∈eff is the dielectric constant (∈r) of the dielectric material proximate the transmission line.
A prior art meanderline, which is one example of a slow wave structure, comprises a conductive pattern (i.e., a traveling wave structure) over a dielectric substrate, overlying a conductive ground plane. An antenna employing a meanderline structure, referred to as a meanderline-loaded antenna (MLA) or a variable impedance transmission line (VITL) antenna, is disclosed in U.S. Pat. No. 5,790,080. The antenna consists of two vertical spaced apart conductors and a horizontal conductor disposed therebetween, with a gap separating each vertical conductor from the horizontal conductor.
The MLA was developed to de-couple the conventional relationship between the antenna physical length and resonant frequency based on the free-space wavelength.
The antenna further comprises one or more meanderline variable impedance transmission lines bridging the gap between the vertical conductor and each horizontal conductor. Each meanderline couplet is a wave transmission line structure carrying a traveling wave at a velocity less than the free space velocity. Thus the effective electrical length of the slow wave structure is considerably greater than it""s actual physical length. Consequently, smaller antenna elements can be employed to form an antenna having, for example, quarter wavelength properties. As for all antenna structures, the antenna resonant condition is determined by the electrical length of the meanderlines plus the electrical length of the radiating structures.
Although the meanderline antenna described above is relatively narrowband in operation, one technique for achieving broadband operation provides for electrically shortening the meanderlines to change the resonant antenna frequency. In such an embodiment the slow-wave meanderline structure includes separate switchable segments (controlled, for example, by vacuum relays, MEMS (micro-electro-mechanical systems), PIN diodes or mechanical switches) that can be inserted in and removed from the circuit by action of the associated switch. This switching action changes the effective electrical length of the meanderline coupler and thus changes the effective length of the antenna and its resonant characteristics. Losses are minimized in the switching process by placing the switching structure in the high impedance sections of the meanderline. Thus the current through the switching device is low, resulting in very low dissipation losses and a high antenna efficiency.
In lieu of removing and adding meanderline segments to the antenna by switching devices as described above, the antenna can be constructed with multiple selectable meanderlines to control the effective antenna electrical length. These are also switched into and removed from the antenna using the switching devices described above.
Consequently, smaller antenna elements can be employed to form an antenna having, for example, quarter-wavelength properties. As for all antenna structures, the antenna resonant condition is determined by the electrical length of the meanderlines plus the electrical length of the radiating elements.
The meanderline-loaded antenna allows the physical antenna dimensions to be reduced, while maintaining an effective electrical length that, in one embodiment, is a quarter wavelength multiple. The meanderline-loaded antennas operate in the region where the performance is limited by the Chu-Harrington relation, that is,
efficiency=FVQ,
where:
Q=quality factor
V=volume of the structure in cubic wavelengths
F=geometric form factor (F=64 for a cube or a sphere)
Meanderline-loaded antennas achieve this efficiency limit of the Chu-Harrington relation while allowing the effective antenna length to be less than a quarter wavelength at the resonant frequency. Dimension reductions of 10 to 1 can be achieved over a quarter wavelength monopole antenna, while achieving a comparable gain.
A meanderline antenna such as described above, offers desirable attributes within a smaller physical volume than prior art antennas, while exhibiting comparable or enhanced performance over conventional antennas. To gain additional benefits from the use of these meanderline antennas, it is advantageous to minimize the space occupied by the antenna and further to provide the antenna at a lower cost through the use of more efficient antenna construction techniques.
In addition to smaller size, antenna designers strive to minimize manufacturing and assembly costs. Thus it is desirable to develop an antenna design that comprises easily reproducible manufacturing steps, minimizes human labor in the manufacturing process and allows easy integration and assembly of the antenna into the final product.
Thus according to the teachings of the present invention, an antenna is constructed from a plurality of dielectric layers, and further includes conductive surfaces thereon serving as the feed, radiating element and the ground plane. The various conductive surfaces are patterned to achieve the desired antenna performance. In certain embodiments of the present invention, inner facing surfaces of the dielectric layers are also patterned with conductive traces to produce the desired antenna characteristics.